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تسجيل مستخدم جديدBig Three Dragons: A [N II] 122 $mu$m Constraint and New Dust-continuum Detection of A $z = 7.15$ Bright Lyman Break Galaxy with ALMA
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We present new Atacama Large Millimeter/submillimeter Array Band 7 observational results of a Lyman break galaxy at $ z=7.15 $, B14-65666 (Big Three Dragons), which is an object detected in [OIII] 88 $rm{mu m}$, [CII] 158 $rm{mu m}$, and dust-continuum emission during the epoch of reionization. Our targets are the [NII] 122 $rm{mu m}$ fine-structure emission line and underlying 120 $rm{mu m}$ dust continuum. The dust continuum is detected with a $ sim $19$ sigma $ significance. From far-infrared spectral energy distribution sampled at 90, 120, and 160 $rm{mu m}$, we obtaine a best-fit dust temperature of $ 40 $ K ($ 79 $ K) and an infrared luminosity of $ log_{10}(L_{rm IR}/{rm L}_odot)=11.6$ ($12.1$) at the emissivity index $ beta = 2.0 $ (1.0). The [NII] 122 $rm{mu m}$ line is not detected. The 3$ sigma $ upper limit of the [NII] luminosity is $ 8.1 times 10^7 {rm L}_odot$. From the [NII], [OIII], and [CII] line luminosities, we use the Cloudy photoionization code to estimate nebular parameters as functions of metallicity. If the metallicity of the galaxy is high ($ Z > 0.4 {rm Z}_odot$), the ionization parameter and hydrogen density are $ log_{10} U simeq -2.7pm0.1$ and $ n_text{H} simeq 50$-$250 {rm cm}^{-3}$, respectively, which are comparable to those measured in low-redshift galaxies. The nitrogen-to-oxygen abundance ratio, $rm{N/O}$, is constrained to be sub-solar. At $ Z < 0.4 {rm Z}_odot$, the allowed $ U $ drastically increases as the assumed metallicity decreases. For high ionization parameters, the $rm{N/O}$ constraint becomes weak. Finally, our Cloudy models predict the location of B14-65666 on the BPT diagram, thereby allowing a comparison with low-redshift galaxies.
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We present new ALMA observations and physical properties of a Lyman Break Galaxy at z=7.15. Our target, B14-65666, has a bright ultra-violet (UV) absolute magnitude, $M_{rm UV}approx-22.4$, and has been spectroscopically identified in Ly$alpha$ with
a small rest-frame equivalent width of $approx4$ AA. Previous HST image has shown that the target is comprised of two spatially separated clumps in the rest-frame UV. With ALMA, we have newly detected spatially resolved [OIII] 88 $mu$m, [CII] 158 $mu$m, and their underlying dust continuum emission. In the whole system of B14-65666, the [OIII] and [CII] lines have consistent redshifts of $7.1520pm0.0003$, and the [OIII] luminosity, $(34.4pm4.1)times10^{8}L_{rm odot}$, is about three times higher than the [CII] luminosity, $(11.0pm1.4)times10^{8}L_{rm odot}$. With our two continuum flux densities, the dust temperature is constrained to be $T_{rm d}approx50-60$ K under the assumption of the dust emissivity index of $beta_{rm d}=2.0-1.5$, leading to a large total infrared luminosity of $L_{rm TIR}approx1times10^{12}L_{rm odot}$. Owing to our high spatial resolution data, we show that the [OIII] and [CII] emission can be spatially decomposed into two clumps associated with the two rest-frame UV clumps whose spectra are kinematically separated by $approx200$ km s$^{-1}$. We also find these two clumps have comparable UV, infrared, [OIII], and [CII] luminosities. Based on these results, we argue that B14-65666 is a starburst galaxy induced by a major-merger. The merger interpretation is also supported by the large specific star-formation rate (defined as the star-formation rate per unit stellar mass), sSFR$=260^{+119}_{-57}$ Gyr$^{-1}$, inferred from our SED fitting. Probably, a strong UV radiation field caused by intense star formation contributes to its high dust temperature and the [OIII]-to-[CII] luminosity ratio.
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